sort of/kind of support PANICTRACE on VMS (trunk only)

I don't think this is useful enough to recommend ordinary users
enable it, but it's close enough to being useful that I don't want
to leave it to become subject to bit rot like umpteen other unfinished
patches.  Anyone running in wizard mode who has a panic already gets
pushed into the debugger on VMS, although it doesn't work for what might
be considered the most important configuration (a secure playground, as
opposed to the wide-open one I've always been content to leave mine at).

include/extern.h

@@ -2486,6 +2486,9 @@ E boolean NDECL(authorize_wizard_mode);
 
 E void NDECL(vms_abort);
 E void FDECL(vms_exit, (int));
+#ifdef PANICTRACE
+E void FDECL(vms_traceback, (int));
+#endif
 
 /* ### vmstty.c ### */